1. Field of the Invention
This invention relates to an elastomeric orthodontic device (such as a ligature) that is resistant to staining.
2. Description of the Related Art
Orthodontic treatment normally involves the application of mechanical forces to urge improperly positioned teeth into correct alignment. One common form of orthodontic treatment includes the use of orthodontic brackets that are fixed to teeth. A resilient curved arch wire, seated in slots of the brackets, is bent or twisted at strategic locations. The restoring force exerted by the arch wire tends to shift the teeth into orthodontically correct alignment.
Various types of elastomeric devices, also known as force modules, are used in orthodontic treatment. For example, tiny O-ring devices are used as ligatures to secure the arch wire in slots of the brackets. Elongated devices, including chain-like modules having a number of interconnected O-ring portions, may be stretched between selected brackets in order to move certain teeth relative to other teeth. Other devices are especially adapted to separate adjacent teeth or to rotate a tooth about its long axis. Examples of such devices can be found in U.S. Pat. Nos. 3,530,583, 3,758,947, 4,038,753 and 4,950,158.
Elastomeric devices for orthodontic use are often clear in color when new, and present a satisfactory appearance when first installed in the mouth next to metallic or ceramic brackets and metallic archwires. However, unlike most brackets and archwires, conventional elastomeric devices quickly stain when exposed to foods or beverages such as mustard, tea, and coffee. Mustard, in particular, has been known as a stain agent that is difficult to resist. Brushing the teeth or rinsing the mouth normally does not remove the stain, and the unsightly appearance persists until the devices are replaced. Discoloration is particularly noticeable when the brackets are made of translucent ceramic and assume the color of the underlying tooth.
U.S. Pat. No. 4,933,418 indicates that a particular crosslinked polyurethane orthodontic device made using polycaprolactone is more resistant to staining by mustard than devices made of other mentioned materials. However, there is a continuing need in the art to provide elastomeric orthodontic devices that are more resistant to staining than the devices known in the past, in order to improve the appearance of the device in the mouth as much as feasible. Elastomeric orthodontic devices should also permit substantial elongation before rupture, be resistant to tearing, and provide satisfactory resistance to force degradation over extended periods of time.